1. Field of the Invention
The present invention relates generally to flight control systems for helicopters and more specifically to a system for controlling the approach of the helicopter to a hover condition at instrument landing system and microwave landing system facilities, without the requirement of sophisticated and expensive low airspeed sensors, Doppler radar, or precision position sensors.
2. Description of the Prior Art
In the area of fixed wing aircraft, low minimum approach fully automatic and semi-automatic control systems have been achieved by the prior art. However, systems of this type are generally too costly and occupy excessive space and weight to be practical for small and medium size helicopters. They are required to be fail-operational in nature, and the attendant redundancy requires extreme sophistication and expense. Moreover, such systems have been certified for low minimum altitude operation only at Category II and Category III ILS airport facilities. Such facilities exist at only a relatively few major airports Helicopters do not typically fly into such major facilities, but rather operate from smaller general aviation type airports. Thus, for a system to be practical in the helicopter market, it must be reasonably simple and economical as well as be operational at Catagory I ILS sites.
The key to successful low altitude approach maneuvers is the ability of the pilot to decelerate down to very low airspeeds at the terminal end of an approach. Flying at extremely low airspeeds (5-10 knots) results in a low descent rate, thereby providing the pilot with sufficient time to assess his situation and apply corrective measures if necessary i.e., continue the approach, take over with manual control, initiate a go-around maneuver, etc.
The key to low speed helicopter operation is the capability of measuring the aircraft speed down to substantially zero values. The accuracy of conventional pitot systems deteriorates rapidly at speeds below approximately 60 knots. This is due in part because of the helicopter rotor blade downwash effects which introduce undesirable inputs into the airspeed measuring system. Systems have been produced which employ mast mounted sensors which can measure low air speeds accurately, but these systems are susceptible to damage and again are too expensive for the commercial helicopter market. Doppler type radar based systems can also measure low speed accurately, but because of their cost and size, have proven to be practical only in military type helicopters.
Some previous systems have employed open loop deceleration techniques which rely on longitudinal accelerometer values only. These systems often compute a desired descent path and speed to a predetermined geographical location based on the initial measurements when entering the glide path. Such calculations of these descent paths will necessarilly require assumptions about the winds that will be encountered during the descent, since their exact nature is unknown. It may be seen, therefore, that winds differing significantly from the assumed values will cause the aircraft to deviate from the desired path and thereby result in either under or over flying the target geographical location. Further, these systems may result in terminal velocities which are unpredictable when operating under adverse wind conditions.
In U.S. Pat. No. 4,551,804, invented by T. R. Clark and C. D. Griffith, and assigned to the assignee of the present invention, a constant deceleration is commanded using a vertical gyro-referenced longitudinal accelerometer as the feedback sensor. The actual deceleration as a time function of the airspeed from the initial airspeed to a second airspeed is measured. Based on these measures, the total deceleration time to approach to a hover condition is predicted, after which the commanded deceleration is removed. However, this system is subject to inaccuracies, when operating under adverse wind conditions.
The present invention provides improved performance without requiring reliance on precision approach radar or distance measuring systems, and provides compensation for departures from the glide slope. Only conventional low-cost on-board sensors are used. The invention computes helicopter airspeed during the terminal end of an approach by making use of parameters already available in a typical helicopter automatic flight control system, such as vertical speed, glide slope error, and longitudinal acceleration. These parameters are blended together in such a manner as to provide a measure of helicopter ground speed, which is then used in a control law computation to decelerate the helicopter to a near hover speed as the vehicle approaches a predetermined altitude minimum. This results in a helicopter positioned at a fixed altitude (typically 50 feet) above the runway at a ground speed near zero. In this situation, the pilot can take over manually and land the aircraft or initiate a go-around maneuver if visibility conditions are too poor to land. Since the system is ground speed rather than acceleration or airspeed based, its accuracy is not dependent on wind conditions